The present invention relates to linear beam tube devices, in particular, to electron beam tube devices such as IOTs and Klystrons.
Linear beam tube devices such as electron beam tube devices are used for the amplification of RF signals. There are various types of linear electron beam tube device known to those skilled in the art, two examples of which are the klystron and the Inductive Output Tube (IOT). Linear electron beam tubes incorporate an electron gun for the generation of an electron beam of an appropriate power. The electron gun includes a cathode heated to a high temperature so that the application of an electric field between the cathode and an anode results in the emission of electrons. Typically, the anode is held at ground potential and the cathode at a large negative potential of the order of tens of kilovolts.
Electron beam tubes used as amplifiers broadly comprise three sections. An electron gun generates an electron beam, which is modulated by application of an input signal. The electron beam then passes into a second section known as the interaction region, which is surrounded by a cavity arrangement including an output cavity arrangement from which the amplified signal is extracted. The third stage is a collector, which collects the spent electron beam.
In an inductive output tube (IOT) a grid is placed close to and in front of the cathode, and the RF signal to be amplified is applied between the cathode and the grid so that the electron beam generated in the gun is density modulated. The density modulated electron beam is directed through an RF interaction region, which includes one or more resonant cavities, including an output cavity arrangement. The beam is focused by a magnetic means, typically electromagnetic coils to ensure that the electron beam passes through the RF region and delivers power at an output section within the interaction region where the amplified RF signal is extracted. After passing through the output section, the beam enters the collector where it is collected and the remaining power is dissipated. The amount of power which needs to be dissipated depends upon the efficiency of the linear beam tube. Efficiency refers to the difference between the power of the beam generated at the electron gun region and the RF power extracted in the output coupling of the RF region.
The difference between an IOT and a Klystron is that in an IOT, the RF input signal is applied between a cathode and a grid close to the front of the cathode. This causes density modulation of the electron beam. In contrast, a klystron velocity modulates an electron beam, which then enters a drift space in which electrons that have been speeded up catch up with electrons that have been slowed down to form bunches of electrons. The bunches are thus formed in the drift space, rather than in the gun region itself.
Linear beam tube devices typically have one of two output arrangements: an external output cavity or an integral output cavity. An external output cavity is one in which the output coupling, typically using a coupling loop, is external to the vacuum envelope of the drift tube interaction region. An integral output cavity is one in which the coupling loop protrudes into the interaction region.
We have appreciated the need to adjust the coupling arrangement of a linear beam tube device. We have further appreciated that this is relatively simple for external cavity devices in which the coupling loop is outside the vacuum envelope, but is problematic for prior integral cavity devices.